In recent years, attention has been given to MIMO (Multiple-Input Multiple-Output) as a technology for implementing high-speed and large-capacity communication in a wireless communication technique. MIMO refers to a technology for transmitting data using a plurality of antennas in both of reception and transmission. This technology enables, without increasing time and frequency resources, an improvement in transmission capacity by transmitting different pieces of data from a plurality of transmission antennas, and is receiving attention as a system for implementing high-speed transmission by utilizing a limited frequency band effectively.
Above all, in multi-user MIMO in which MIMO channels are formed by a plurality of users, transmission beam control is applied, or more specifically, among specified beam patterns, a beam corresponding to feedback from each user is used, thereby allowing signal strength to be ensured at the time of transmission to the relevant user. Moreover, a different beam can accommodate other user, and a multi-user diversity effect capable of improving transmission efficiency is obtained by appropriately selecting a user for simultaneous transmission.
Furthermore, at present, 3GPP (3rd Generation Partnership Project), which is an international standardization organization for mobile phones, is conducting standardization activities for an LTE (Long Term Evolution) system as a system for implementing communication speed and capacity higher than those of the current third-generation mobile phones. Also in this LTE, MIMO takes its place as an essential technique in order to meet conditions required for high-speed and large-capacity transmission. Besides, in this LTE, a transmission beam technique is discussed as a “Pre-coding” (hereinafter referred to as “preceding”) technique.
FIG. 13 is a diagram schematically illustrating data transmission carried out by multi-user MIMO. For example, at the time of multi-antenna transmission from a wireless base station 101 for use in a cellular system for communication of mobile units such as mobile phones, data is transmitted to first user equipment (UE-A) 102 and second user equipment (UE-B) 103, each serving as a mobile station, with the use of beams suitable to the respective equipments. In this case, a beam number indicative of an optimal beam pattern among a plurality of beams is set for each piece of user equipment 102 and 103 based on its reception quality, and this beam number and a CQI (Channel Quality Indicator) indicative of the reception quality are fed back to the wireless base station 101. In the example of FIG. 13, data is transmitted to the user equipment (UE-A) 102 using a beam 2, and data is transmitted to the user equipment (UE-B) 103 using a beam 3.
Further, in the cellular system to which multi-user MIMO is applied, studies are conducted on power scaling for changing power distribution between users, thereby ensuring the signal strength of a cell edge user located at a peripheral edge of a cell.
FIG. 14 provides diagrams schematically illustrating operations to be performed at the time of power scaling in multi-user MIMO, in which (A) is a diagram illustrating beams emitted to respective user equipments and (B) is a diagram illustrating power allocated to the respective user equipments. In the example of FIG. 14, the user equipment (UE-A) 102 is located in the vicinity of the wireless base station 101, and the user equipment (UE-B) 103 is located at a cell edge. Furthermore, power scaling is carried out to allocate a large amount of power to the user equipment (UE-B) 103 located at the cell edge and to reduce power allocated to the adjacent user equipment (UE-A) 102, with the overall power kept constant. In this case, a transmission signal S transmitted from the wireless base station 101 to the user equipment (UE-B) 103 increases in power and thus becomes an interference signal I for the user equipment (UE-A) 102, resulting in an increase in the level of the interference signal I for the user equipment (UE-A) 102. Accordingly, the influence of interference between users, which has been reduced by a beam in multi-user MIMO, is increased, and therefore, the rate of transmission to the user equipment in the vicinity of the base station is reduced.    Non-Patent Document 1: 3GPP TSG RAN WG1 #42, R1-050912, Qualcomm Europe, “MIMO proposal for MIMO-WCDMA evaluation”, Aug. 29-Sep. 2, 2005    Non-Patent Document 2: 3GPP TSG RAN WG1 #42, R1-050724, Texas Instruments, “MIMO OFDMA Techniques for Downlink E-UTRA”, Aug. 29-Sep. 2, 2005    Patent Document 1: JP-A-2003-78461